Originally posted by orangbaligila
Sorry to jump in,.. but isn't it the other way around?
I usually richen it when ambient temp is hot to cool it down. and lean it when temp is cold to build more heat and reach "optimum" working temp...
Its all about mixture. If your mixture is already correct to begin with then in theory, warm ambient temperature means that there is less oxygen. To bring your mixture back to the right ratio of oxygen-fuel then you need to lean the engine.
When ambient temperature is cooler, you tend to have "heavy" air and more oxygen which is why you need to richen your engine to bring your mixture back to the correct ratio.
Now that I have moved from Asia to the UK (i.e. warm, tropical climate to a cooler climate) I am a firm convert of this theory. All my engines which were running great in Asia now needed to be re-tuned. Otherwise, as soon I start them up they are running so lean and overheating like mad!